This invention relates to the field of electron beam lithography systems, and more particularly to an exposure system for a multiple electron beam lithography system.
As integrated circuits become more complex there is a general trend in the semiconductor industry toward increasing device packaging density. The present philosophy is to keep chip or die sizes as small as possible, and so increase device yield. However, it is obvious that chip size cannot be arbitrarily reduced because of the inherent resolution limits of presently-used photolithographic processes. In particular, the wavelength of light imposes a barrier to the reproduction of detail in the region of one micron.
A number of solutions have been proposed to solve the resolution problem and there is considerable activity in this area presently underway in the semiconductor industry. These solutions are based on lithographic techniques that employ particles of shorter wavelengths than visible light to overcome the resolution limitation. Generally, two classes of short wavelength particles have been proposed as alternatives to light, i.e., high energy photons (X-rays) and electrons.
The currently known lithographic systems are either based on the use of X-rays or electron beams. There are two basic types of electron beam systems. One type of electron beam system uses modified scanning electron microscope techniques and the other uses projection techniques.
An optimal lithographic exposure system would have to possess certain attributes in order for it to be a serious contender for use in the production of integrated circuits. These attributes include resolution, coverage, lithographic speed, reregistration and stability.
At present, minimum reproducible line widths of 1 micron are mandatory, and future integrated circuit structures will probably push the desired system resolution requirements to below 1/4 micron. The exposure system should be capable of covering the standard 3" and/or 4" wafers presently used in industry and be expandable to cover the larger sized wafers being contemplated. Clearly wafer exposure times measured in tens of minutes to hours are not acceptable in production exposure systems. Acceptable throughput conditions demand full 3" and/or 4" wafer exposure times of at most a few minutes.
The multilayer structure of integrated circuits necessitates that there be a reregistration mechanism built into the exposure system. In general, reregistration capabilities should be a minimum of a factor of five times better than the size of the smallest detail required. Another consequence of multilayer structure necessitates that even with an acceptable reregistration scheme, the lithographic system should be stable during the exposure period so that the registration will remain precise over the entire surface of the wafer.
Comparing the three known lithographic techniques noted above, the X-ray system has been demonstrated to have more than enough resolution; however, exposure times are still on the order of tens of minutes. Furthermore, no completely acceptable reregistration method seems to exist for X-rays at the moment. Stability is also a problem with X-ray systems because the masks that are used are essentially thin pellicles of material (mylar or silicon) which are temperature and pressure sensitive. Finally, although coverage of areas about 2" in diameter have been demonstrated, expansion to larger areas appears to present formidable difficulties.
Scanning electron beam methods have fewer limitations than the projection systems. Scanning systems have been demonstrated with the necessary resolution, coverage, stability, and reregistration capabilities. The main problem appears to be one of exposure time. For example, some current exposure systems take approximately 60 minutes to expose a 3" wafer. Although this time is adequate for mask production, it is far from being useful for use in direct write on-line production systems. Present research indicates that with improved electron guns and more sensitive electron resists the exposure times may be reduced to less than about 15 minutes. However, exposure times in the range of 2 to 3 minutes will be needed for production-oriented systems.
The most promising lithographic technique seems to be the scanning electron beam system provided the limitation of long exposure time can be overcome. The potential usefulness of the scanned electron beam technique is more apparent when it is noted that both X-ray and electron projection techniques require a high resolution mask and this mask must initially be made by a scanning electron beam system. Finally, there is the additional advantage of scanning systems that the master "mask" exists as data stored on tape or disc where it can be easily and rapidly updated. This capability eliminates the need for a physical mask and thus avoids any problems attributable to wear and subsequent deterioration of the mask.
A multiple image electron beam exposure system has been suggested by Westerberg, in U.S. Pat. No. 3,619,608, wherein an array of electron lenses is used to produce an array of closely spaced demagnified images of an aperture mask on a target surface without requiring physical displacement. The array of patterns is therefore written faster by a function directly related to the number of lenses. However, it should be apparent that certain inherent limitations of the disclosed system would preclude its effective use in connection with production equipment. In particular the small field of view of usable beam deflection, before the onset of electron optical aberrations, would limit the effective die size to an area far smaller than that required for integrated circuit purposes.
It is accordingly a general object of the present invention to overcome the aforementioned limitations and drawbacks associated with known systems and to fulfill the needs mentioned by providing an exposure system for a charged particle beam lithography system having all of the desirable attributes noted above.
It is a particular object of the invention to provide a high resolution exposure system having a high throughput capability.
It is a further object of the invention to provide an exposure system for a multiple charged particle beam lithography system for utilization in the production of integrated circuits.
Other objects will be apparent in the following detailed description and the practice of the invention.